Atmospheric Moisture

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Atmospheric Moisture

• Chapter 5

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Objectives

• Explain the concept of saturation
• Compare several indices of water vapor
• Compare several ways of measuring water vapor
• Examine 3 methods of creating saturation

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Water vapor and phase change

• Evaporation molecules break free of liquid and
  become vapor
• Condensation water molecules come together to
  become a liquid
• Sublimation water goes directly from ice to
  vapor without going through the liquid phase
• Deposition water goes directly from vapor to
  ice

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Water Vapor - saturation

• Point at which rate of evaporation equals
  condensation

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Indices of Water Vapor

• Vapor pressure part of the atmospheric pressure
  caused by water vapor
• Saturation vapor pressure the maximum water
  vapor that can exist at any temperature
• Figure 5-3, page 137

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Figure 5-3, page 137
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Indices of Water Vapor

• Absolute humidity density of water grams of
  water in m3 of air
• Specific humidity mass of water per mass of air
• Saturation specific humidity maximum specific
  humidity

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Indices of Water Vapor

• Mixing ratio mass of water vapor / mass of all
  other gasses in the atmosphere
• Saturation mixing ratio maximum possible mixing
  ratio

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Indices of Water vapor

• Relative humidity amount of water vapor in the
  air relative to the maximum possible at that
  temperature
• RH (specific humidity/saturation specific
  humidity) X 100

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Indices of Water Vapor

• Relative humidity

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Indices of Water Vapor

• Dew point the temperature at which the air will
  become saturated.
• If air temp is above the dew point the air is
  unsaturated
• If air temperature is equal to the dew point the
  air is saturated
• Air temperature can not be below the dew point

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Indices of Water Vapor

• Dew-point

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Measuring Humidity

• Sling psychrometer
• Aspirated psychrometer
• Hair hygrometer

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Find the dew point
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Condensation

• Homogeneous nucleation water molecules collide
  and form droplets if the air is supersaturated
• Heterogeneous nucleation water molecules adhere
  to hygroscopic condensation nuclei

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Condensation below -4C

• Requires 6-sided freezing nuclei

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Achieving Saturation

• Adding water vapor
• Mixing cold air with warm moist air
• Lowering the temperature

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Adding water vapor

• Evaporation of water from moist surfaces or
  bodies of water
• Precipitation fog forms from evaporation of
  raindrops

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Mixing air

• Fig. 5-7 warm moist air can mix with cool air to
  produce air at the dew point

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Lowering the temperature

• Diabatic processes
• Adiabatic processes

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Diabatic processes

• Energy is added or removed from a system
• Air passing over a cool surface can cool to the
  point of saturation

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Adiabatic processes

• Temperature changes, but no energy is added or
  removed
• Expanding air cools
• Air being compressed warms

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Dry Adiabatic Lapse Rate DALR

• As air rises through the atmosphere it expands
  and thus cools
• Cools at a rate of 10C for every 100m

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DALR
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Wet Adiabatic Lapse Rate

• Condensation releases latent heat
• Rising saturated air cools slower than dry air
• Not a linear lapse rate

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The Environmental Lapse Rate

• The actual change in temperature in temperatures
  as one moves upward through the atmosphere
• Plotted in a Stüve diagram

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Types of condensation

• Get these from the book

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Objectives

• Explain the concept of saturation
• Compare several indices of water vapor
• Compare several ways of measuring water vapor
• Examine 3 methods of creating saturation

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Have a Excellent Rest of the Day!